A magnetic tunnel junction (MTJ) device is comprised of two ferromagnetic layers separated by a thin insulating tunnel barrier layer and is based on the phenomenon of spin-polarized electron tunneling. One of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed as memory cells for solid state memory and as external magnetic field sensors, such as magnetoresistive (MR) read sensors or heads for magnetic recording systems. The use of a MTJ device as a memory cell in a nonvolatile magnetic random access memory (MRAM) array is described in IBM's U.S. Pat. Nos. 5,650,958 and 5,640,343.
The response of the MTJ device is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, a ferromagnetic layer whose magnetic moment is fixed or prevented from rotation, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material comprising the ferromagnetic layer at the interface of the ferromagnetic layer with the tunnel barrier layer. The first ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the second ferromagnetic layer. Usually, when the magnetic moment of the second ferromagnetic layer is parallel to the magnetic moment of the first ferromagnetic layer, there are more available electronic states than when the magnetic moment of the second ferromagnetic layer is aligned antiparallel to that of the first ferromagnetic layer. The tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. Thus, the electrical resistance of the MTJ depends on both the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers.
For a memory cell application one of the ferromagnetic layers in the MTJ has its magnetic moment fixed or pinned so as to be either parallel or antiparallel to the magnetic moment of the other free or sensing ferromagnetic layer in the absence of an applied magnetic field within the cell. It is thus necessary to be able to distinguish the two states of the MR memory device, i.e., parallel and antiparallel alignment of the moment of the free ferromagnetic layer with the moment of the fixed ferromagnetic layer. However, if when changing the state of the memory cell, the magnetic moment of the free layer is switched from one direction to the opposite direction by the formation of magnetic domains or inhomogeneous magnetic structures in the free layer, the speed of writing the new state of the cell is decreased compared to the case in which the moment of the free layer is uniformly rotated from one direction to the other without the formation of magnetic domains. Moreover, when magnetic domains are formed in the free layer it is possible that not all of these magnetic domains are removed when the magnetic fields applied during the writing of the new state of the cell are removed. Thus the magnitude of the moment of the free layer may be reduced after its state has been re-written, which can lead to a diminishment of the difference in resistance between the two states of the cell and in a consequent increase in the time required to read the state of the cell with the same precision. If some means is not used to stabilize the free ferromagnetic layer of the MTJ, i.e., to maintain it in a single magnetic domain state, then the performance of the memory cell will be degraded with respect to both writing and reading.
The problem of maintaining a single magnetic domain state is especially difficult in the case of a MTJ memory cell because, unlike a MTJ MR sensor, it is not possible to add additional magnetic structures adjacent to the device without reducing the density of the memory array. Thus the method of longitudinally biasing a MTJ MR device, which is described in IBM's U.S. Pat. No. 5,729,410, by adding electrically isolated ferromagnetic biasing regions outside the MTJ stack is not suitable for high density memory applications.
What is needed is a MTJ memory cell that has two reproducible magnetic states in which the magnetic moment of the free ferromagnetic layer is either parallel or antiparallel to the magnetic moment of the fixed ferromagnetic layer and for which the magnetic moment of the free layer rotates coherently between these two states.